1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly, to a measurement method and apparatus of a User Equipment (UE) that has a Discontinuous Reception (DRX) cycle and is in a Radio Resource Control (RRC) connected mode.
2. Description of the Related Art
The Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation asynchronous mobile communication system that uses Wideband Code Division Multiple Access (W-CDMA) based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), both of which are European mobile communication systems.
In 3rd Generation Partnership Project (3GPP), which is in charge of UMTS standardization, discussions are being conducted regarding the Long Term Evolution (LTE) system as the next generation mobile communication system of the UMTS system. The LTE system is a technology for realizing high-speed packet-based communication having a maximum data rate of 100 Mbps, and aiming at commercialization in around 2010. Several communication schemes are under discussion for the LTE system. For example, a first scheme reduces the number of nodes located in the communication path by simplifying a configuration of the network, and a second scheme maximally approximates radio protocols to a radio channel.
FIG. 1 illustrates a configuration of an LTE system to which the present invention is applicable.
Referring to FIG. 1, an Evolved Radio Access Network (E-RAN) 110 is expected to be simplified to a 2-node configuration of Evolved Node Bs (ENBs) 120, 122, 124, 126 and 128, and anchor nodes 130 and 132. A User Equipment (UE or ‘terminal’) 101 can access an Internet Protocol (IP) network by means of the E-RAN 110, for communication.
The ENBs 120-128, corresponding to the existing Node Bs of the 3rd generation system, can communicate with the UE 101 over the radio channel. The ENBs 120-128, compared to the existing Node Bs, will perform complex functions. This is due to the fact that in the LTE system, user traffic including real-time IP services such as Voice over IP (VoIP) will be serviced over a shared channel. Therefore, there is a need for an apparatus for collecting status information of multiple UEs and performing scheduling depending thereon, which is expected to be managed by the ENBs 120-128. Each ENB can control a plurality of cells.
The ENBs 120-128 perform Adaptive Modulation & Coding (AMC) that adaptively determines a modulation scheme and a channel-coding rate according to the channel status of the UE. In addition, the ENBs 120-128, together with the UE 101, will perform the Hybrid Automatic Repeat reQuest (HARQ) technique used in the services of the 3rd generation system, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), and Enhanced Dedicated Channel (E-DCH). When various Quality of Service (QoS) requirements cannot be satisfied with only the HARQ technique, the ENBs 120-128 and the UE 101 will apply an Outer ARQ technique in their upper layers.
It is expected that the LTE system will use Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology in the 20-MHz bandwidth.
FIG. 2 illustrates a scheduling operation of a UE having a DRX interval, to which the present invention is applicable.
In the LTE system, operation modes of a UE can be classified into two modes: a Radio Resource Control (RRC) idle mode and an RRC connected mode. The RRC idle mode refers to the UE's status when there is no UE context information and service context information in the ENB. Further, the anchor node manages locations of the UEs using the UE context information, not in units of cells, but in units of tracking areas for paging. The RRC connected mode refers to the status when not only the anchor node but also the ENB have the UE context information, and the ENB can manage locations of the UEs, having an established RRC connection, in units of cells. The possibility that the service context information will also be included is not excluded herein.
In other words, an arbitrary UE in the RRC idle mode (hereinafter referred to as an ‘RRC idle mode UE’) first establishes an RRC connection to the ENB to receive or transmit the data for a particular service. In this case, the RRC idle mode UE should provide the UE context information to the ENB, and should establish a signaling connection even to the anchor node, It should also provide the UE context and service context information to the anchor node over the signaling connection. As a result, a UE in the RRC connected mode (hereinafter referred to as an ‘RRC connected mode UE’) can be immediately allocated corresponding radio resources from the ENB, and receive or transmit the data for a particular service using the allocated radio resources.
Referring to FIG. 2, the general RRC connected mode UE continuously receives a scheduling channel, and if the scheduling channel includes scheduling information for the UE itself, the RRC connected mode UE receives or transmits the data over the corresponding radio resources indicated by the scheduling channel. Herein, the UE determines whether UE identifier information (UE ID) included in the scheduling channel is identical to its allocated unique UE identifier information (unique UE ID), to check the presence of scheduling information for the UE itself.
In the LTE system, the RRC connected mode UE can perform a Discontinuous Reception/Transmission (DRX/DTX) operation to minimize its battery power consumption. That is, during the DRX interval (or cycle), the UE receives no scheduling channel associated with the data or control information. During the DRX interval, the UE disables (or turns OFF) its transmission and reception operations and operates in a sleep state.
Therefore, the RRC connected mode UE having the DRX interval discontinuously receives a scheduling channel and checks the presence/absence of scheduling information for the UE itself. For example, a UE having a DRX interval previously agreed upon between the ENB and the UE, enters the sleep state during the DRX interval to minimize the power consumption. After expiration of the DRX interval, the UE can transition to a wake-up state (or awake state) and receive a scheduling channel. The operation in which the UE discontinuously receives a scheduling channel using the DRX interval can include (i) an operation in which the DRX interval is determined according to a hard-coding scheme, (ii) an operation in which the DRX interval is determined according to a predefined rule such as a DRX interval calculation formula, and (iii) an operation in which the UE is allocated the DRX interval from the ENB by signaling.
Reference numerals 201, 202, 203 and 204 of FIG. 2 show the times at which the UE receives a scheduling channel from the ENB and checks the presence/absence of scheduling information for data reception/transmission.
If the UE has received a scheduling channel and the received scheduling channel includes no scheduling information for the UE itself, the UE transitions back to the sleep state for the DRX interval.
For example, the UE in the sleep state receives no channel transmitted over the downlink. In other words, the ENB does not perform scheduling for the UE during the DRX interval even though it has generated data for the UE to acquire scheduling synchronization with the UE.
If the ENB performs scheduling for the RRC connected mode UE having the DRX interval at time 204, the ENB, after the time 204, may stop the application of the DRX interval and operate in the RRC connected mode where DRX is disabled. That is, after the ENB has started the scheduling at the time 204, the ENB continuously schedules the UE without applying the DRX interval, and the UE can continuously receive a scheduling channel.
FIG. 3 illustrates a measurement operation of a UE having a DRX interval. The UE, as it performs measurement using the DRX interval, can minimize the power consumption.
The UE checks whether the scheduling for data reception/transmission has been performed at the time that it receives a scheduling channel, and at this time, the UE performs the measurement as well. Therefore, the UE performs no measurement for the DRX interval where it enters the sleep state, thereby reducing the power consumption, compared to when it continuously performs the measurement.
Referring to FIG. 3, the UE performs measurement on the current serving cell and neighboring cells at times 301, 302, 303, 304 and 305, and performs no measurement for the DRX interval.
After performing the measurement at time 301, the UE determines that there is no neighboring cell whose radio strength is greater than or equal to that of the current serving cell.
After performing the measurement at time 302, if the UE determines that there is a neighboring cell whose radio strength is greater than or equal to that of the serving cell, the UE starts (or activates) a counter or a timer for triggering measurement report. The measurement report is an operation in which the UE provides the ENB with information on a change in the radio environment. Based on the measurement report, the UE can perform handover. For example, upon detecting the presence of a neighboring cell having a greater radio strength than the current serving cell, the UE provides the ENB with information on a cell ID and radio strength of the neighboring cell. After checking the cell ID and the radio strength, the ENB determines whether to hand over the UE to the neighboring cell. The counter or the timer is used for preventing the frequent transmission of a measurement report due to the frequent change in the radio environment.
Generally, the measurement report for the changed radio environment cannot be transmitted by the UE to the ENB as long as a greater or equal radio strength is not continuously maintained during the count of the set counter or the time of the set timer. In other words, the UE transmits a measurement report to the ENB, when the state where the neighboring cell has radio strength greater than or equal to that of the serving cell is continuously maintained during the measurement count of the counter or the time of the timer. The measurement count or time is set immediately after the UE detects the presence of the neighboring cell whose radio strength is greater than or equal to that of the serving cell. For example, in FIG. 3, the counter is set to a value of 3. The counter value is subject to change. Therefore, upon detecting the neighboring cell having greater radio strength than that of the serving cell at time 302, the UE transmits a measurement report to the ENB in step 321 when the neighboring cell continuously has the greater radio strength than that of the serving cell during the set 3 measurement counts 311 of 303, 304 and 305.
As described in FIG. 3, the UE has a long delay time until it actually transmits the measurement report to the ENB. That is, the measurement report operation employing the counter or timer for preventing the frequent transmission of a measurement report, or for reducing the unnecessary signaling overhead, cannot actually transmit the measurement report at an appropriate time.
Therefore, the UE and the serving cell, which cannot perform the correct measurement report operation and cannot perform handover at an appropriate time, may bring a failure of the radio link between them. The radio link failure prevents normal transmission/reception not only of the control information necessary for data transmission/reception between the UE and the serving cell, but also of the actual data.
For example, assuming that a length of the DRX interval is 5.12 sec, the UE performs a measurement report operation having a long delay time of a total of 5.12 sec*3 intervals=15.36 sec, from the time 302 where the UE has determined that the neighboring cell has radio strength being greater than or equal to that of the current serving cell until the time 305 where the UE transmits the measurement report.
The reason the long delay time problem and the resulting problems occur is due to the fact that the UE's measurement operation performs the measurement only once every 5.12-sec DRX interval in the LTE system, as shown in FIG. 3. The UE cannot normally support the scenario where it should undergo handover for the DRX interval due to the change in the radio environment during the DRX interval.